system-variables: Add charset documentation (#5867)

[ti-chi-bot]

This is an automated cherry-pick of #5867

What is changed, added or deleted? (Required)

Add documentation for these:

5.7.25-TiDB-v5.2.0-alpha-164-g8434069c5 127.0.0.1:4000  test  SQL  show global variables like 'char%';
+--------------------------+--------------------------------------------------------+
| Variable_name            | Value                                                  |
+--------------------------+--------------------------------------------------------+
| character_set_client     | utf8mb4                                                |
| character_set_connection | utf8mb4                                                |
| character_set_database   | utf8mb4                                                |
| character_set_filesystem | binary                                                 |
| character_set_results    | utf8mb4                                                |
| character_set_server     | utf8mb4                                                |
| character_set_system     | utf8                                                   |
| character_sets_dir       | /usr/local/mysql-5.6.25-osx10.8-x86_64/share/charsets/ |
+--------------------------+--------------------------------------------------------+
8 rows in set (0.0042 sec)

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

Related to #3155

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 024c450